Distribution of Incompatible Trace Elements between the Constituents of Mantle Spinel Peridotites: Inversion of ICP-MS Data

Abstract

Minerals of the Earth's upper mantle have crystal structures notoriously unfavourable to large ion lithophile elements (LILE, such as K, Rb, Cs, Ba, Th, U), and to some extent to the high field strength elements (HFSE, such as Nb and Ta). Several authors have suggested that, in the convective mantle and the lower lithosphere, these 'highly incompatible elements' are dominantly concentrated in small fractions of partial melt. These melts would be volatile-rich and rather similar in composition to kimberlites and/or carbonatitic magmas (McKenzie, 1989). Alternatively, at shallow depth (lithosphere), the small volume melts could be silica-rich (Schiano and Clocchiatti, 1994; Kelemen et al., 1995). In the lithospheric mantle, LILE and HFSE may also be concentrated in accessory minerals such as amphibole, phlogopite, apatite and titanates, in fluidderived inclusions trapped in minerals, or along grain boundaries (e.g. Zindler and Jagoutz, 1988). Among the predominant rock-forming minerals, only clinopyroxene is considered to play a significant role as a host for incompatible elements. However, very few high-quality are available to constrain these assumptions. Minerals such as orthopyroxene, olivine and spinel have rarely been analysed, and the importance of fluid/melt inclusions and grain boundaries as reservoirs of trace elements remains controversial. For this reason, some important issues such as the behaviour and distribution of the ItFSE in the upper mantle are still a matter of debate. The aim of this paper is to provide quantitative estimates of the distribution of lithophile trace elements between the various constituents of spinel peridotites from the East African Rift. Predominant and accessory minerals were separated in 12 mantle xenoliths from Mega (Sidamo region, South-Eastern Ethiopia). The samples range in composition from fertile, cpx-rich lherzolites to refractory harzburgites Dipartimento di Scienze della Terra e Geologico-Ambientali, Universitg di Bologna, Piazza di Porta S. Donato I, Bologna 140127, Italy